Search for the charged Higgs boson in the etau and mutau dilepton channels of top quark pair decays

The large top quark production cross section at the Large Hadron Collider offers the opportunity to search for the charged Higgs boson as the decay $t\rightarrow H^+b\rightarrow \tau^+\nu b$ can be large in extensions of the standard model, if the mass is lighter than the top quark mass. A direct search for the charged Higgs boson in events with one lepton (electron or muon), at least two jets, missing transverse energy, and one hadronically decaying tau lepton has been performed in 36~pb$^{-1}$ of data collected in proton-proton collisions at $\sqrt{s}=7$~TeV. We do not find a signal in our search and derive an upper limit on the branching ratio $BR(t\rightarrow H^+b)\simeq 0.25-0.28$ for masses between 80 and 140~GeV\!/\!$c^2$, when assuming $BR(H^+\rightarrow \tau^+\nu)=1$.